Our objective is to clarify the mechanism of action of the H-2 region genes which control the serum levels of the Slp (sex-limited protein) and Ss (C4). The first goal is to determine whether structural or regulatory genes are involved. For this purpose we will search for polymorphisms of Slp and Ss among inbred and outbred strains of mice by using a modification, recently developed in our laboratory, of the two-dimensional O'Farrell technique. It involves subjecting the radiolabelled, partially purified protein to immunoprecipitation with a monospecific antiserum, followed by isoelectrofocussing under denaturing conditions, SDS-polyacrylamide gel electrophoresis in a second dimension, and radioautography. As described in the application, we have already detected an H-2 controlled variation of gamma chain of Slp. We will also use the same method to screen mouse serum for genetic variations of mouse complement components which regulate the activity of C3-convertase; that is, C4-bp, C3bINA and beta 1H. These proteins interact with C3 and C4 and may themselves be under H-2 control. Two of the control proteins (C4-bp and C3bINA) have recently been isolated from mouse serum in our laboratory, and we are currently involved in the purification of a third, i.e., beta 1H. A related objective is to clarify whether the Slp protein is part of the complement system, and to search for a homologous protein in human serum.